Add CoroutineReturn and CoroutineExit builtin exceptions for coroutines

[1st1]

BPO	24697
Nosy	@gvanrossum, @ncoghlan, @scoder, @1st1

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/1st1'
closed_at = <Date 2016-10-05.23:48:17.715>
created_at = <Date 2015-07-23.15:10:15.634>
labels = ['interpreter-core', 'type-feature']
title = 'Add CoroutineReturn and CoroutineExit builtin exceptions for coroutines'
updated_at = <Date 2016-10-05.23:48:17.713>
user = 'https://github.com/1st1'

bugs.python.org fields:

activity = <Date 2016-10-05.23:48:17.713>
actor = 'yselivanov'
assignee = 'yselivanov'
closed = True
closed_date = <Date 2016-10-05.23:48:17.715>
closer = 'yselivanov'
components = ['Interpreter Core']
creation = <Date 2015-07-23.15:10:15.634>
creator = 'yselivanov'
dependencies = []
files = []
hgrepos = []
issue_num = 24697
keywords = []
message_count = 8.0
messages = ['247214', '247217', '247218', '247227', '247229', '247230', '247263', '278164']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'ncoghlan', 'scoder', 'yselivanov']
pr_nums = []
priority = 'normal'
resolution = 'wont fix'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue24697'
versions = ['Python 3.6']

[1st1]

Since native coroutines (see PEP-492) hadn't had a separate type when the PEP was accepted, we didn't discuss that it might be necessary to introduce new exception types specifically for coroutines.

To maintain backwards compatibility with 3.5, I think we can do the following:

1. Add CoroutineReturn exception type, inherited from StopIteration;

2. Add CoroutineExit exception type, inherited from GeneratorExit.

[gvanrossum]

What problem does this solve?

[gvanrossum]

What problem does this solve?

[1st1]

What problem does this solve?

Only avoiding confusion, because coroutines now have a separate type, lack __iter__, and thus are quite different (on the surface) from generators. The fact that 'coro.send(..)' raises StopIteration (when coroutines aren't iterable), and that 'coro.close()' raises GeneratorExit might be confusing and non-obvious to some users.

FWIW I created this ticket mostly as a reminder for myself to have some discussion on this topic in the future, when 3.5 is released and we have some initial feedback on PEP-492 ideas.

[gvanrossum]

Hm, I think there's little need for new exceptions...

[scoder]

Hm, I think there's little need for new exceptions...

While I agree with Yuri that the names are a bit awkward, I actually second this. The StopIteration is almost an implementation detail of how the return value is passed on to become the (Future) result of the coroutine. It is intended to be caught by some framework and users would not normally get to see that exception. There is little room for general confusion.

[ncoghlan]

My initial inclination is to agree with Stefan. At the moment, we have a
slightly leaky abstraction where the exceptions used mean that coroutines
still expose the fact that under the covers they're defined in terms of
generator semantics.

However, that leak in the abstraction reveals an underlying truth -
coroutine semantics *are* derived from generator semantics, and they *do*
share common underlying infrastructure.

We may eventually find pragmatic reasons for wanting to plug that leak and
use separately named exceptions, but unlike the situation with coroutines
themselves, I'm not currently seeing a clear gain in either usability or
comprehensibility as a payoff for the extra complexity.

[1st1]

Closing this one. Seems we can live just fine without these new exceptions.